Conventionally, an apparatus using a transfer member transport belt as transfer member transfer means is well known as an image-forming apparatus of this type as shown in FIG. 1.
In FIG. 1, numeral 1 designates a photosensitive drum, numeral 2 a charger, numeral 3 an exposure device, numeral 4 a developer, numeral 5 a transfer charger, numeral 6 an erase lamp, numeral 7 a cleaner, numeral 8 a belt cleaner, numeral 9 a transfer member, numeral 10 a transfer member transport belt, and numerals 11, 12 a belt roller.
The photosensitive drum 1 has arranged therearound various parts including the charger 2, the exposure device 3, the developer 4, the transfer charger 5, the erase lamp 6 and the cleaner 7. The transfer member 9, which is transported by the transfer member transport belt 10, is supplied to a nip region where the photosensitive drum 1 is in contact with the transfer member transport belt 10.
This image-forming apparatus performs the operation described below.
First, the surface of the photosensitive drum 1 is uniformly charged by the charger 2, and is irradiated with light corresponding to the desired image to be formed at the exposure device 3 thereby to form an electrostatic latent image on the surface of the photosensitive drum 1. In the next step, toner is attached on the electrostatic latent image in the developer 4, so that a visible image is formed on the photosensitive drum 1. This visible image is moved to the nip region. The transfer member 9, on the other hand, is supplied from the direction A as indicated by arrow, and disposed on the transfer member transport belt 10 followed by being transported to the nip region by the transfer member transport belt 10. In the nip region, since the charge of a polarity opposite to that of the visible image is applied to the transfer member transport belt 10 from the transfer charger 5, the visible image on the photosensitive drum 1 is transferred onto the transfer member 9 in the nip region. The transfer member 9 with a visible image transferred onto the surface thereof, after being transported over a predetermined distance by the transfer member transport belt 10, is moved along the direction B as indicated by arrow separated from the transfer member transport belt 10, followed by an image being fixed on the surface of the transfer member 9 by means of a fixer (not shown).
The residual charges on the photosensitive drum 1 are erased by the erase lamp 6, and then the residual toner on the photosensitive drum 1 is removed by the cleaner 7 in preparation for a new image-forming cycle. The toner attached on the transfer member transport belt 10, on the other hand, is removed by the belt cleaner 8.
In the image-forming apparatus as mentioned above, by the way, the transfer member 9 that has passed a transfer point may usually be undesirably wound on the photosentive drum 1 due to various causes as a result of a phenomenon of what is called drum trap.
In the prior art, type, means for preventing drum trap is added. A typical one of such means is an eraser such as an AC corotron disposed at the boundary between the transfer member 9 and the transfer member transport belt 10 to enable the transfer member 9 to easily come off from the transfer member transport belt 10.
Further, as shown in FIG. 1, before transferring a visible image onto the transfer member 9, the transfer member transport belt 10 and the transfer member 9 thereon are charged into opposite polarities by a spare charger 14 and a conductive brush 15 conductively connected with the spare charger 14. As a result, the transfer member 9 is adsorbed electrostatically to the transfer member transport belt 10, thereby preventing the transfer member 9 from being wound on the photosentive drum 1, i.e., the drum trap from occurring. This function is disclosed in JP-A-1-274173.
To achieve the same object, as shown in FIGS. 2A and 2B, a holder 16 with or without a conductive brush 18 grounded conductively through a constant-voltage element 17 is arranged in contact with the transfer member transport belt 10. When the transfer member 9 is supplied, the holder 16 with or without the conductive brush 18 is operated to press the transfer member 9 onto the transfer member transport belt 10. This apparatus is disclosed in JP-A(U)-62-127561.
Another apparatus whose object is different from that of the above-mentioned apparatuses and is to clean the transfer member transport belt is disclosed in JP-A-62-275942, in which a scratch-off blade is arranged in the direction perpendicular to the progress of the transfer member transport belt over the whole belt width thereof to remove paper dust attached on the transfer member transport belt. Still another apparatus having the same object of cleaning the transfer member transport belt is described in JP-A(U)-63-78964 in which a blade element is arranged at an angle to the joint of the transfer member transport belt over the whole effective belt width thereby to clean the surface of the transfer member transport belt.
The means disclosed in JP-A-1-274173 and the one disclosed in JP-A(U)-62-127561 have the problem mentioned below in spite of the fact that both of them have the function to prevent drum trap, i.e., the transfer member 9 from winding on the photosensitive drum 1.
More specifically, in an image-forming apparatus of this type, the transfer member 9 of various sizes or shapes is often selected in accordance with the image to be transferred. Since the transfer member transport belt 10 is preliminarily charged over the whole width thereof regardless of the shape or size of the transfer member 9, the toner which may drop on the transfer member transport belt 10 is stored uniformly over the whole width thereof.
In this case, the toner stored on the transfer member transport belt 10 is attached to the transfer member 9 that has been transported on the transfer member transport belt 10, and with each movement of the transfer member 9, is removed from the surface of the transfer member transport belt 10. If a large transfer member 9 is used after continuous use of a comparatively small transfer member 9, the toner portion that has failed to be removed from the surface of the transfer member transport belt 10 adheres to the forward end of the large transfer member 9 at once, thereby contaminating the the image to be formed.
Conventionally, an electrophotographic recording apparatus equipped with means for preventing the occurrence of the drum trap phenomenon (hereinafter referred to as "the anti-trap means"), as disclosed in JP-A-1-121879, JP-A-1-121878 and JP-A-1-172986, has a separate close-contacting corona charger arranged on a recording paper transporter upstream of the location of a transfer corona charger. The recording paper is brought into close contact with the recording paper transporter immediately before transfer to the recording paper by the close-contacting corona charger (hereinafter referred to as "the close-contact type"). According to another well-known electrophotographic recording apparatus disclosed in JP-A-55-9505, the recording paper is charged to the polarity opposite to that of the recording paper transporter immediately before being placed on the recording paper transporter, so that the recording paper and the recording paper transporter are adsorbed to each other when the former is placed on the latter (hereinafter referred to as "the quasi-close-contact type"). Still another conventional electrophotographic recording apparatus is disclosed in JP-A-61-5256, according to which a recording paper transporter is formed with a plurality of pores, and the recording paper is adsorbed to the recording paper transporter through the pores by a vacuum adsorber after transfer to the recording paper (hereinafter referred to as "the adsorption type"). The recording paper transporter used in the above-mentioned conventional apparatuses is required to hold the charges generated at a corona charger and is comprised of a single- or plural-layer member of plastic film such as urethane rubber or PVDF.
Investigation by the present inventors has revealed that the drum trap occurred in an electrophotographic recording apparatus is caused by the following factors:
First, in the case where no toner image is formed at the forward end portion of the recording paper or especially in the region within at least 20 to 30 mm of the forward end, i.e., when the same region is a blank printing region, the drum trap is liable to occur. When a toner image is formed in the same region, in contrast, the drum trap phenomenon rarely occurs as the result of interposition of toner between the photosensitive member and the recording paper.
Secondly, in the-case where the recording paper is cut paper in the size of A4 or A3 which is obtained by cutting a larger paper into predetermined sizes of paper, what is called a burr occurs with the cut portion (forward end) of the recording paper bent in the cut direction. Also, the forward end at other than the cut portion of the recording paper may be bent or folded for some reason or other. A large burr or bend at the forward end of the recording paper is liable to cause the drum trap. Especially when a burr or bend is transported directed toward the recording paper transporter, the probability of occurrence of the drum trap is considerable.
Thirdly, the frequency of occurrence of the drum trap phenomenon depends on the type of the recording paper, e.g., thickness or quality thereof (whether a high quality paper or recycled paper), the size of a burr or bend being almost the same. Drum trap is liable to occur especially easily with weak paper such as thin or recycled paper. This is also the case with the OHP (overhead projector) sheet such as plastic film used as transfer paper as well as ordinary paper.
Fourthly, the occurrence of drum trap depends on the environmental conditions, and is liable to occur more frequently with a decrease in humidity. Especially when humidity is as low as less than 30% RH, the probability of drum trap is high.
As explained above, the probability of occurrence of the drum trap depends on various factors. In view of this, recent demand is for anti-trap means which is free of the effects of variations in these factors.
All the conventional anti-trap means described above are not designed paying due consideration to these factors. Also, the above-mentioned conventional anti-trap means of close-contact or quasi-close-contact type requires a close-contacting corona charger or a charging roller in addition to a transfer corona charger. Further, the anti-trap means of close-contact type has the problem that a visible image corresponding to a predetermined region at the forward end of the recording paper cannot be transferred. Furthermore, the conventional anti-trap means of adsorption type requires a vacuum adsorber, and has the problem that it is difficult to set the timing of adsorbing the recording paper.